The present invention relates to an antenna arrangement comprising a multiband antenna having at least one feed point and a multiplexer for connection between the antenna and a transceiver. The present invention further relates to a radio communications apparatus incorporating such an arrangement. In the present specification, the term multiband antenna relates to an antenna which functions satisfactorily in two or more distinct frequency bands but not in the unused spectrum between the bands.
Multiband radio communications apparatuses are becoming increasingly common. For example, cellular telephones are available which can operate in GSM (Global System for Mobile Communications), DCS1800 and PCS1900 (Personal Communication Services) networks. Future apparatus is likely to operate in an even greater range of networks. Implementation of such apparatus requires the availability of multiband antennas and transceivers capable of driving such antennas.
It is conventional for a multiband antenna to be realised as a multi-resonant single feed antenna. There are two common ways of achieving antenna multi-resonance. The first is by having different parts of the antenna structure resonate at different frequencies, for example by the use of two antennas joined at a common feed point. The second is by integrating a transmission line matching structure within the antenna with distributed capacitance and inductance to realise a multi-band matching circuit.
A multiband antenna is normally fed via a multiplexer having one input per frequency band and a single output. The function of the multiplexer is to provide isolation between the various inputs and to provide a known impedance at the inputs which are not in use for a particular frequency band. The multiplexer output drives the antenna via antenna matching circuitry, which must therefore be effective over all frequency bands. The matching circuitry may also perform a broadbanding function, to enhance the bandwidth available from compact antennas such as planar antennas.
A problem with the conventional multiband antenna arrangement described above is that the antenna matching has to be effective at a plurality of frequencies. The more frequencies that are to be matched the more difficult this becomes, which means that the opportunity for other optimisations, such as bandwidth enhancement, is lost.
An object of the present invention is to provide a multiband antenna arrangement having improved performance.
According to a first aspect of the present invention there is provided an antenna arrangement comprising a multiband antenna having at least one feed point and a multiplexer, the multiplexer comprising at least one input, at least one output and isolation means, the or each output being coupled to a respective antenna feed point, wherein the or each coupling between an antenna feed point and a multiplexer output has a substantially negligible impedance.
By ensuring that the coupling between the antenna and the multiplexer is not influenced by parasitic or other ill-defined discrete components (for example circuit board track impedances), it is ensured the isolating function of the multiplexer is not compromised. The negligible impedance would typically be ensured by implementing the multiplexer and antenna close to one another, possibly on the same substrate. For an antenna having a plurality of feed points, implementation of the multiplexer close to the feed points enhances the isolation between the feed points.
An antenna arrangement made in accordance with the present invention enables the use of antennas having multiple feeds, which has the advantage of allowing the isolation of the feeds from one another and also of allowing individual matching of the feeds. By implementing some or all of the matching between the antenna and a transceiver within the multiplexer, it is possible to have independent matching and bandwidth broadening for each frequency band. As well as being much easier to implement than multiple frequency matching and bandwidth broadening, it allows further bandwidth enhancement via resonant matching circuitry. Further improvements and economies can be realised by sharing of components between matching, bandwidth broadening and multiplexing functions.
According to a second aspect of the present invention there is provided a radio communications apparatus including an antenna arrangement made in accordance with the present invention.
The present invention is based upon the recognition, not present in the prior art, that by having the multiplexer located close to the antenna no significant impedances are present between the antenna and multiplexer. The resultant antenna arrangement has improved performance and is simpler to design than prior art arrangements.